The invention relates to a method and an apparatus for forming annular doses, in particular for forming an annular dose made of plasticised material supplied by an extruder and applying the dose that has just been formed to a surface.
Specifically, but not exclusively, the invention can be used to apply a seal to a surface, for example to form seals inside capsules (made of metal or plastics) to close containers. The invention can be further used to apply an annular dose to a flat element (for example a disc), made in particular of metal or plastics that can then be inserted, as a sealant, into a more complex device. It is possible, in other uses of the invention, to apply the annular dose (acting as a sealant) to a cardboard element intended, for example, to form a container or a portion of a container, or directly to a container, for example made of plastics or metal. The annular dose could also be inserted, according to another use of the invention, directly into a mould to obtain an object therefrom (for example a cap consisting of a wall of plastics and a central metal disc provided with a seal obtained from the annular dose) or also with different functions from the closing function.
The prior art comprises the patent publication US 2012/0171381 A1, which shows an apparatus made in accordance with the preamble to claim one, for forming an annular dose of plasticised material to be deposited on the surface of a cap for containers to give rise to the sealing washer. In such an apparatus the dose is separated and expelled by virtue of the great closing velocity of the mould that makes the material squeeze out.
This known apparatus nevertheless has numerous limits and drawbacks.
Firstly, the material that forms the annular dose is pressed out at high pressure, with a possible localised increase of the temperature and consequent deterioration of the material and/or adhesion of the material to the conduit surfaces.
Secondly, in order to press out the material, low viscosity has to be maintained and it is thus necessary to work at relatively high temperatures.
Further, whilst on the one hand using high-viscosity materials causes very high working pressure, on the other hand using low-viscosity materials causes problems of leakage of the material from the seals.
Another drawback is that the annular dose, by squeezing out at high speed from the mould, can get deformed in a manner that is not easily controllable.